Carbon Dioxide Redox Flow Battery Having a Bi-Functional Negative Electrode

ABSTRACT

A redox flow battery (“CRB”) performs as an energy storage system and has a negative electrode that directly utilizes CO2 in the battery charge step as an active species instead of metals. The CRB also has a positive electrode utilizing a metallic or non-metallic redox species, and a cation exchange membrane in between the negative and positive electrodes. The negative electrode comprises a porous base layer, a porous intermediate layer containing a metal oxide and a bi-functional catalyst layer for electrochemical reduction of CO2 or carbonate to formate and for formate oxidation to either carbonate or CO2. The bi-functional catalyst can be a PdSn based catalyst, such as PdSn, PdSnIn, and PdSnPb. The metal oxide in the intermediate layer acts as a catalyst support and can be a non-Platinum group metal (PGM) oxide, such as LaCoO3 or LaNiO3.

FIELD

This disclosure relates generally to carbon dioxide flow batteries, andin particular, to a negative electrode of a carbon dioxide redox flowbattery that directly utilizes CO₂ as an active species instead ofmetals, and a method of manufacturing same.

BACKGROUND

Carbon capture and utilization (CCU) for the production of value-addedchemicals and fuels is intensely researched by all possible catalyticpathways: thermo, bio, photo, electrocatalytic routes and combinationsthereof. The electrocatalytic option is attractive due to the increasingdeployment of renewable electricity sources and their decreasing cost ofgeneration (e.g. solar and wind). However, the scale of the CO₂ emissionproblem is so enormous that it has been estimated the power-to-fuels andchemical production routes could only utilize between 1% and up to about10% of the annual CO₂ emissions by 2050.

Therefore, there is a need for additional, novel CO₂ utilizationtechnologies that address decarbonization. A new wave of electrochemicaltechnologies proposed recently is focused on the utilization of CO₂ inprimary and secondary (rechargeable) batteries. In this pathway, CO₂either alone or in combination with other species (e.g. O₂) is anelectroactive species in the battery. Thus, CO₂ batteries could addresssimultaneously and in a flexible manner the storage of intermittentrenewable energy sources and the utilization of CO₂ captured fromindustrial emission sources. Proposed systems thus far, used CO₂ inconjunction with a metal negative electrode in metal-CO₂ batteries with:Li, Al, Zn, Mg or Na. The performance of these metal-CO₂ batteries todate is lower as compared to other metals batteries including lithium.Ion, vanadium redox flow or Zn-air.

It is therefore an object to provide a new and improved CO₂ battery tocurrent metal-CO₂ batteries.

SUMMARY

According to one aspect of the invention, there is provided a redox flowbattery comprising a negative electrode, a positive electrode and acation exchange or bipolar membrane in between the negative and positiveelectrodes. The negative electrode comprises a porous base layer, abi-functional catalyst layer for electrochemical reduction of either CO₂or carbonate to formate during battery charging and for formateoxidation to either carbonate or CO₂ during battery discharge, and anintermediate support layer supporting the bi-functional catalyst layerand comprising a metal oxide. The metal oxide has either: a perovskitestructure with the general formula AxByO₃, wherein A is one or a mixtureof La, Sr, and Ba and B is one of Co, Ti, Fe, Ni, Ga, Mg, In, Mn, Ta, orCe; or a fluorite structure with the general formula AxByO₇, wherein Ais Nd. and B is Ir.

The bi-functional catalyst layer can comprise one or more of Pd, Sn, anintermetallic species with the formula Pd_(x)Sn_(y), SnO₂, In and Pb.The bi-functional catalyst layer can be electrodeposited or ink sprayedon the intermediate support layer. The bi-functional catalyst layer canfurther comprise polytetrafluoroethyelene (PTFE) and one or more carbonadditives selected from a group consisting of: carbon black, graphene,and carbon nanotubes, with a PTFE to carbon additive weight ratiobetween 0.1:1 to 5:1.

The intermediate support layer can comprise LaCoO₃ electrodeposited orink sprayed onto the porous base layer. The porous base layer can be ateflonated carbon cloth or a carbon fiber paper. The intermediatesupport layer can further comprise PTFE and carbon additives selectedfrom a group consisting of: carbon black, graphite and grapheneparticles, with a PTFE to carbon additive weight ratio between 0.1:1 to5:1.

The metal oxide in the intermediate support layer can comprise LaCoO₃mixed with MnO₂. In such case, the intermediate support layer cancomprise LaNiO₃ electrodeposited or ink sprayed on the porous baselayer.

The intermediate support layer can further comprise silicon with thegeneral formula AxBySiO₄, wherein A is one of Mg, Zr, and Ca, and B isone of Fe and Ni. Alternatively, the metal oxide in the intermediatesupport layer can comprise one of Ce, Zr, Al, and Ga.

According to another aspect of the invention, there is provided a methodfor electrochemically activating the negative electrode of the redoxflow battery claimed in claim 1, comprising electrode potential sweepingbetween reduction and oxidation potentials or current pulsing betweenreduction and oxidation currents.

According to yet another aspect of the invention, there is provided amethod for manufacturing a bi-functional negative electrode for a redoxflow battery, comprising: providing a porous carbon base layer;providing a deposition mixture for an intermediate support layercomprising a metal oxide material having a perovskite structure with theformula AxByO₃, wherein A is one or a mixture of La, Sr, and Ba and B isone of Co, Ti, Fe, Ni, Ga, Mg, in, Mn, Ta, or Ce; or a fluoritestructure with the formula AxByO₇, wherein A is Nd, and B is Ir; and,providing a deposition mixture for a bi-functional porous catalystlayer. The intermediate support layer deposition mixture is depositedonto the carbon base layer by electrodeposition or mechanical spraying,and then the catalyst layer comprising the bi-functional catalyst isdeposited onto the intermediate layer by electrodeposition or mechanicalspraying, creating a metal oxide supported catalyst.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a)-(c) are schematic views of a non-metal CO₂ redox flow battery(“CRB”) according to one embodiment, wherein FIG. 1(a) shows a chargingcycle (energy storage) of the CRB, FIG. 1(b) shows a discharge cycle(energy generation) of the CRB and FIG. 1(c) is an exploded perspectiveview of a single electrochemical cell of the CRB.

FIGS. 2(a)-(e) are images and graphs for an experimental CRB having abi-functional intermetallic PdSn catalyst electrodeposited on teflonatedcarbon fibre cloth, wherein FIG. 1(a) is a SEM image, FIG. 2(b) are EDXmapping images, FIG. 2(c) is a XRD spectrum, and FIGS. 2(d) and (e) arecyclic voltammograms of electrodeposited Pd (FIG. 2(d)) and PdSn (FIG.2(e)) samples in CO₂ saturated 0.5 M KHCO₃+0.1 M KHCO₂ (pH of 7.4) andN₂ saturated 0.5 M KHCO₃ (pH adjusted to 7.4), with pureelectrodeposited Pd (FIG. 2(d)—loading 10.7 mg cm⁻²) and PdSn (FIG.2(e)-loading 6.6 mg cm⁻²).

FIGS. 3(a)-(d) are graphs of experiments showing galvanostaticpolarization cycling of the CRB the negative electrode in half-cellexperiments having electrodeposited PdSn bi-functional catalysts,wherein FIG. 3(a) shows a the electrode potential during galvanostaticcycling of a negative electrode composed of only teflonated carbon fibercloth base layer and bi-functional catalyst (without the intermediatelayer containing the non-PGM support), and FIG. 3(c) shows a negativeelectrode with a LaCoO₃ (loading of 0.5 mg cm⁻²) intermediate layer onteflonated carbon fiber cloth acting as support for the electrodepositedPdSn catalyst. FIGS. 3(b) and (d) are cyclic voltammograms ofelectrodeposited PdSn catalyst in the FOR potential region after 1 hr ofFOR (formate oxidation reaction) at 50 mA cm⁻² in 0.1 M KHCO₂+0.5 MKHCO₃ followed by 1 hr of CO₂RR (CO₂ reduction reaction) at −35 mA cm⁻²in CO₂ saturated 0.5 M KHCO₃. FIG. 3(e) is a cyclic voltammogram of widepotential range electrochemical activation test of the negativeelectrode with PdSn catalyst and LaCoO₃ support consisting of 50 cyclesin N₂ saturated 0.5 M KHCO₃ starting from −2000 mV to 1100 mV at 20 mVs⁻¹. FIG. 3(f) shows a galvanostatic polarization cycling of thenegative electrode with activated LaCoO₃ support-PdSn catalyst with theFOR—CO₂RR tests repeating for 18 cycles.

FIGS. 4(a)-(e) are graphs of experiments showing the bi-functionalperformance of a CRB negative electrode in half-cell experimentscomparing the binary PdSn with the ternary PdPbSn and PdInSn catalystsfor CO₂RR and FOR, wherein FIG. 4(a) shows a galvanostatic polarizationcycling on the electrodeposited PdPbSn and PdSnIn catalysts withelectrochemical activation applied, FIG. 4(b) shows galvanostaticpotentials for CO₂ electro-reduction at −20, −35 and −50 mA cm⁻² and 30min. on activated bimetallic and ternary electrocatalyst, and FIGS.4(c),(d) and (e) show net formate faradaic efficiency (FE) and netformate production rates on geometric electrode area and catalyst massbasis, wherein catalyst loadings are: PdSn 1.4 mg cm⁻² (FIG. 4(c)),PdPbSn 8.1 mg cm⁻² (FIG. 4(d)) and PdSnIn 3.6 mg cm⁻² (FIG. 4(e))

FIG. 5 shows a single-cell CRB configuration used for small-scaleelectrode screening experiments.

FIGS. 6(a) and (b) are graphs of experimental results showing the effectof LaCoO₃ intermediate support layer and electrochemical activation onthe single-cell CRB charge and discharge polarization withelectrodeposited PdSn bi-functional negative electrode catalyst attemperatures of 293 K (FIGS. 6(a)) and 318 K (FIG. 6(b)). Negolyte: 2 MKHCO₃+1 M KHCO₂. Posolyte: 0.3 M Br₂+2 M KBr. Membrane: Nafion 115. CO₂flow rate: 3.17×10⁻³ L min⁻¹ (SLM), 1 atm. Current sweep rate: 0.1 mAs⁻¹. Temperature: a) 293 K and b) 318 K. Catalyst loading: PdSn 6.6 mgcm-2. Polarization curves are ohmic potential drop-corrected.

FIGS. 7(a)-(b) respectively show discharge/charge polarization curves ofthe single-cell CRB at 293 K and 318 K, and FIGS. 7(c) and (d)respectively show discharge power density curves at 293 K and 318 Kusing gas diffusion negative electrodes with LaCoO₃ supported andactivated PdSn and PdSnPb electrodeposited bi-functional catalysts,respectively. Negolyte: 2 M KHCO₃+1 M KHCO₂ (pH of 8.03). Posolyte: 0.3M Br₂+2 M KBr. Membrane: Nafion 115. CO₂ flow rate: 3.17×10-3 standard Lmin-1 (SLM). Current sweep rate: 0.1 mA s-1. At 293 K and 318 K.Catalyst loadings: PdSn (6.6 mg cm⁻²) and PdSnPb (8 mg cm⁻²). The solidand dashed lines refer to 100% and 0% ohmic potential drop correction,respectively.

FIGS. 8(a)-(f) are graphs showing results from state of charge tests andgalvanostatic polarization cycling on a single-cell CRB with LaCoO₃supported and activated PdSn electrodeposited bi-functional catalysts,compared to Sn and Pd electrodeposited catalysts, respectively, whereinFIGS. 8 (a), (c) and (e) show state of charge tests starting withopen-circuit potential (OCP) measurements followed by three stages ofcharging at 20 mA cm⁻² for 1 hour each and OCP-measurement breaks of 15min. in 2 M KHCO₃ (negolyte) at 293 K, and FIGS. 8 (b), (d) and (f) showgalvanostatic polarization cycling on the same electrodes at ±0.5 mAcm⁻² (5 min each).

FIG. 9 is a graph showing the power density of the single-cell CRBequipped with an electrochemically activated gas diffusion negativeelectrode prepared by spraying the PdSn catalyst layer onto the LaCoO₃intermediate layer, which in turn was prepared by spraying onto the baselayer. Base layer: 40 wt % Teflon (PTFE)-treated carbon cloth;Intermediate layer: sprayed LaCoO₃:Vulcan XC-72:Nafion:PTFE at a weightratio of 1:1:0.6:0.63; Catalyst layer: sprayed Pd/C:SnO₂ activated (1:1weight ratio, loading of 2 mg cm⁻² each). Negolyte: 2 M KHCO₃+1 M KHCO₂(pH of 8.03). Posolyte: 0.3 M Br₂+2 M KBr. Membrane: Fumasep FKD-PK-75.CO₂ flow rate: 3.17×10⁻³ standard L min⁻¹ (SLM). Current sweep rate: 0.1mA s⁻¹. No ohmic drop compensation is applied.

FIG. 10 is a schematic diagram showing a negative electrode of the CRBcomposed of three layers: Base transport layer (referred to also asgas-diffusion layer), non-PGM oxide intermediate layer and thebi-functional catalyst layer.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments described herein relate generally a redox flow battery thatperforms as an energy storage system and has a negative electrode thatdirectly utilizes CO₂ as an active species instead of metals (hereinreferred to as “non-metal CO₂ redox flow battery” or “CRB”), a positiveelectrode, and a cation exchange or bipolar membrane in between thenegative and positive electrodes. In some embodiments, the CRB at thenegative electrode uses an electrocatalytic reduction of either CO₂ orcarbonate to formate (HCOO⁻) during a charge step and HCOO⁻electrocatalytic oxidation during a discharge step (i.e. powergeneration) (eq. 1 and 2). The negative electrode comprises a porousbase layer, a porous bi-functional catalyst for electrochemicalreduction of CO₂ or carbonate to formate and for formate oxidation toeither carbonate or CO₂, and a metal oxide containing porousintermediate layer acting as a support for the bi-functional catalyst.In some embodiments, the metal oxide support is a non-Platinum groupmetal (PGM) oxide support, such as LaCoO₃, or LaNiO₃. In some otherembodiments, the metal oxide comprises LaCoO₃ mixed with MnO₂, or LaNiO₃mixed with MnO₂. The metal oxide support can have a perovskite structurewith the general formula AxByO₃, wherein A is one or a mixture of: La,Sr, and Ba, and B is one of: Co, Fe, Ti, Ni, Ga, Mg, In, Mn, Ta, or Ce.The metal oxide support can additionally contain silicon with thegeneral formula AxBySiO₄, wherein A is one of Mg, Ca, and B is one ofFe, Ni. In some other embodiments, the metal oxide support has afluorite structure with the general formula AxByO₇, wherein A is Nd, andB is Ir. Furthermore, in some other embodiments, the metal oxide supportadditionally contains one of Ce, Al, and Ga, and for example, can be oneof CeO₂, BaCeO₃, Ga doped CeO₂, Al₂O₃, Ag doped Al₂O₃ and Ga₂O₃.

In order to accomplish the CO₂/HCOO⁻ interconversion, efficientbi-functional (or bi-directional) electrocatalysts are required tominimize the activation overpotential losses on the negative electrodeduring battery charge and discharge, respectively. For the positiveelectrode, either metallic or non-metallic redox species could beutilized. Among the latter, halide/polyhalogen redox couples (e.g.,X′X2⁻, where X′ and X are Br and/or I) can be used due to fast andreversible electrode kinetics on cost-effective, carbon-basedelectrodes. More particularly, Br⁻/Br2 can be used at the positiveelectrode. Complexing agents can be added (e.g., quaternary ammoniumsalts) to promote the formation of tri-bromide species and lower the Br2vapor pressure.

In some embodiments of the CRB, an electrocatalytically reversibleCO₂/formate redox couple is employed at the negative electrode and ahalide/poly-halide redox couple (e.g. Br⁻/Br₃ ⁻ or I⁻/I₃ ⁻) is employedat the positive electrode as shown in equations (1)-(3), to provide abattery charge cycle as shown in FIG. 1(a) and equation (4) and abattery discharge cycle as shown in FIG. 1(b) and equation (5):

Negative Electrode:

Under the pH conditions explored in this study (i.e., between 7 and 8)the negative electrode reactions of the rechargeable battery are:

$\begin{matrix}{{CO}_{2,{(g)}} + {H_{2}O_{(l)}} + {2{e^{-}\overset{{charge}\mspace{14mu}\rightarrow{discharge}\mspace{14mu}\leftarrow}{\Longleftrightarrow}{HCOO}_{{({aq})}^{-}}}} + {{HO}_{{({aq})}^{-}}.}} & (9)\end{matrix}$

According to the Nernst equation and considering ideal gas and solution,the equilibrium potential corresponding to the negative electrodeaccording to eq. 1 is given by:

$\begin{matrix}{{E_{e,T,{( - )}} = {E_{T,{( - )}}^{0} - {\frac{RT}{2F}\ln\frac{C_{HCOO^{-}}}{p_{CO2}}} + {\frac{2.3{RT}}{2F}\left( {{pK_{w}} - {pH}} \right)}}},} & (2)\end{matrix}$

where E⁰ _(T,(−)) is the standard potential (V_(SHE)) for CO₂/HCOO⁻(i.e., activities for all the species equal to one) at temperature T. At298 K and pH of 14 (i.e., OH⁻ activity approximately equal to one), andE⁰ _(298,(−))=−0.64 V_(SHE). Further in eq. 2, C_(HCOO) ⁻ is the formatemolar concentration (M), p_(CO2) is the CO_(2,(g)) partial pressure(atm) and pK_(w) is the water auto-ionization constant (expressed on thedecimal −log scale) at temperature T and total pressure P (in thepresent work 1 atm). R and F are the universal gas constant and Faradayconstant, respectively.

At pH=8 (typical pH condition here, see further), for 1 atm CO_(2,(g)),298 K and formate concentrations of 1 M and 0.1 M, the equilibriumpotential of the negative electrode based on eq. 2 is −0.46 V_(SHE) and−0.43 V_(SHE), respectively.

Furthermore, it is noted that due to the well-known pH-dependentsolubility of CO₂ and HCO₃ ⁻/CO₃ ²⁻ speciation, there are additionalvariants of operation for CRB. In one of these variants, when the CRBnegative electrode is operated at a higher pH during discharge thancharge (e.g., pH˜14 (discharge) and pH˜8 (charge step)), duringdischarge instead of regenerating CO₂ (eq. 1, which would correspond toa closed-loop carbon-neutral operation) carbonate salts can be producedas by-products of electricity generation (eq. 1a):

$\begin{matrix}{{{HCOO}^{-} + {3{OH}^{-}}}\overset{discharge}{\rightarrow}{{CO}_{3}^{2 -} + {2H_{2}O} + {2{e^{-}.}}}} & \left( {1a} \right)\end{matrix}$

The latter variant (referred to as the open-loop) adds a multiplier inthe battery deployment by combining energy storage with CO₂mineralization, therefore, the battery acting as a net CO₂ sink(carbon-negative operation).

FIG. 1 presents schematically the: a) charging step of the CRB, b) thedischarge step coupled with carbonate generation (pH>11), and c) thebasic components of an individual battery cell. The mineralization withcarbonate formation variant was not further pursued here, since thenegative electrode operating pH throughout was between 7 and 8 (withrelevance to eq. 1).

Positive Electrode:

$\begin{matrix}{{Br}_{2,{(l)}} + {2{e^{-}\overset{{charge}\mspace{14mu}\leftarrow{{discharge}\mspace{14mu}\rightarrow}}{\Longleftrightarrow}2}{Br}_{({aq})}^{-}}} & (3)\end{matrix}$

The equilibrium potential considering ideal solutions and molarconcentrations of the species is expressed as:

$\begin{matrix}{{E_{e,T,{( + )}} = {E_{T,{( + )}}^{0} - {\frac{RT}{2F}\ln\frac{C_{{Br}^{-}}^{2}}{C_{Br_{2}}}}}}.} & (4)\end{matrix}$

The standard potential at 298 K, E⁰ _(298K,(+)) is equal to 1.09V_(SHE). For molar concentrations of 0.3 M Br₂ and 2 M Br⁻, theequilibrium potential is 1.13 V_(SHE) (at 298 K).

Battery (CRB) Reactions:

$\begin{matrix}{{CO}_{2,{(g)}} + {H_{2}O_{(l)}} + {2{{Br}_{({aq})}^{-}\overset{{charge}\mspace{14mu}\rightarrow{discharge}\mspace{14mu}\leftarrow}{\Longleftrightarrow}{HCOO}_{{({aq})}^{-}}}} + {HO}_{{({aq})}^{-}} + {Br}_{2,{(l)}}} & (5)\end{matrix}$

The discharge equilibrium cell potential can be obtained from eqns. 2and 4:

E _(e,cell,T) =E _(e,T,(+)) −E _(e,T,(−))  (6)

At 298 K, for one exemplary set of conditions (pH 8, 1 M HCOO⁻, 1 atmCO₂, 0.3 M Br₂, 2 M Br), the battery equilibrium potential is 1.59 V.The corresponding thermodynamic (theoretical) specific energy of the CRBnormalized per mass of reactants in the discharge mode (eq. 5 with K⁺counter ion as per the present study) is 284 Wh kg⁻¹, which is overthree times higher than for the vanadium redox flow battery at 89.8 Whkg⁻¹. While it is understood that the thermodynamic specific energy hasno practical implication, it shows nevertheless, the proposed batterychemistry has a high thermodynamic capability for large-scale energystorage based on the mass of reactants involved. The theoreticalvolumetric energy density is dependent on the solubility of potassiumformate and as a result it varies: between 85.2 Wh L⁻¹ _(formate soln.)(for 1 M KHCO₂ solution) and 3,353.4 Wh L⁻¹ _(formate soln.) (forsaturated 39.35 M KHCO₂ solution at 298 K).

The practical power and energy densities will be dependent on a numberof complex interdependent factors related to electrodes andelectrocatalysts, cell design, membrane performance (e.g., ionicconductivity, selective permeability), electrolyte composition (e.g.,solubility of different species, ionic conductivity), and mass transfer(including two-phase (gas/liquid) flow during battery charge).

Embodiments described herein relate to a CRB that provides forbi-functional electrocatalysis of CO₂/formate interconversion such asintermetallic catalyst compositions and perovskite catalyst support, inorder to decrease the activation overpotentials on the negativeelectrode in both reaction directions.

According to one embodiment and referring to FIGS. 1(a)-(c), a CRB 10comprises a negative electrode with a bi-functional (i.e.bi-directional) catalyst incorporated into the catalyst layer. Asuitable bi-functional catalyst can be a PdSn-based catalyst, such asactivated PdSnPb. Other suitable catalysts include PdSn and PdSnIn. Thecatalyst is supported by a metal oxide support to enhance the overallbi-functional electrocatalytic activity of the catalysts for bothbi-functional CO₂ or carbonate reduction (“CO2RR”) and formate oxidationreaction (“FOR”), mainly by mitigating the effect of CO as a poisoningintermediate. A suitable metal oxide support is a non-Platinum groupmetal (“PGM”) support, such as LaCoO₃ which is part of the intermediatelayer situated between the catalyst layer and the porous base layer. Insome embodiments, the catalyst and the intermediate layer contain Teflon(polytetraflurorethylene, PTFE) to impart partial hydrophobic characterin order to improve the CO₂ gas mass transfer to the catalyst layerduring battery charge and avoid liquid electrolyte flooding of thenegative electrode. Furthermore, in some embodiments, both the catalystand the intermediate layer contain carbon-based additives such as carbonblack (e.g., Vulcan XC72, Ketjenblack), graphitized carbon, graphene,graphene oxide, reduced graphene oxide, carbon nanotubes alone or in acombination, in order to increase the electronic conductivity of theoxide based intermediate layer, enhance the utilization of the catalystlayer and contribute to mass transfer enhancement toward and from thecatalyst layer during battery charge and discharge. In some embodiments,the catalyst layer and/or the intermediate support layer can have acomposition comprising PTFE and one or more carbon additives such ascarbon black, graphene, and carbon nanotubes, having a PTFE to carbonadditive weight ratio between 0.1:1 to 5:1

A CRB 10 equipped with an activated LaCoO₃-supported PdSn electrode isexpected to provide superior performance compared to other emergingbattery technologies such as S-air and CO₂/CH₄—Zn batteries. Suitablemetal oxide supports include: oxide supports with perovskite structurewith the general formula: AxByO₃, wherein A is one of: La, Sr, and Ba,and B is one of: Co, Ti, Ni, Ga, Mg, In, Mn, Fe, Ta and Ce; and oxidesupport with a fluorite structure with the general formula AxByO₇,wherein A is Nd, and B is Ir. In addition to the perovskite or fluoritestructure oxides, the intermediate support layer can contain siliconwith the general formula AxBySiO₄, where A is one of Mg, Ca, and B isone of Fe, Ni; cerium such as CeO₂, BaCeO₃, Ga doped CeO₂; and aluminumsuch as Al₂O₃ and Ag doped Al₂O₃; and gallium such as Ga₂O₃.

As can be seen in FIG. 1(c), the CRB 10 is composed of electrochemicalcell components including a negative electrode assembly 12 comprising agas flow channel 1, an embedded current collector 2, a porous transportbase layer referred to also as a gas-diffusion layer (GDL) 3, anintermediate support layer 4, and a bi-functional catalyst layer 5. TheCRB 10 also includes a cation exchange or bipolar membrane 6, and apositive electrode 7 comprising porous carbon (or graphite).

In some embodiments, the current collector 2 can be a stainless steel ortitanium mesh, the porous base layer 3 can be a teflonated (i.e., PTFEtreated) carbon fiber paper such as Freudenberg H23C7 or carbon cloth,which acts as a gas diffusion and mass transport layer, the intermediatesupport layer 4 can be a sprayed or electrodeposited layer containingthe non-PGM oxide support mixed with carbon and PTFE, and the catalystlayer 5 can be a sprayed or electrodeposited layer containing thebi-functional catalyst such as but not limited to PdSn, carbon andTeflon additives. For example, the intermediate support layer 4 cancomprise LaNiO₃ electrodeposited or ink sprayed on the porous base layer3. The base layer (or GDL) 3 together with the intermediate supportlayer 4 and catalyst layer 5 serve as a gas diffusion negative electrode(GDE). In some embodiments, the positive electrode 7 can be aPTFE-treated carbon fiber layer in 0.3 M Br₂+2 M KBr (posolyte). Thecation exchange membrane 6 can be selected from commercially availablecation exchange membranes, such as Nafion 115.

According to some embodiments, the gas diffusion negative electrode ofthe CRB 10 can be manufactured as follows: a PTFE-treated carbon fibercloth (e.g., 40% wt. PTFE) can be used as the base layer for electrodepreparation. A nitric acid (HNO₃) pre-treatment is performed on thecarbon fiber base layer to increase its surface roughness andwettability before the electrodeposition of the bi-functional catalystlayer. The concentration of the HNO₃ solution, pre-treatment temperatureand duration will have a synergistic effect on the resultant propertiesof the base layer impacting the subsequent spraying or electrodepositionsteps. Next, the intermediate layer is sprayed onto the base layer usinga spray ink comprising of (but not limited to); non-PGM oxide,carbon-based additive and PTFE with a typical weight ratio of 1:1:1.Spraying of the intermediate layer can be carried out in one single stepfollowed by drying or multiple sequential spraying steps with dryingstages in between. Typical loading of the non-PGM oxide is between 0.1to 2 mg cm⁻². Following the intermediate layer application, anelectrodeposition bath is prepared containing (but not limited to) theprecursor chemicals for the bi-functional catalyst material. Diverseelectrodeposition techniques can be used to deposit the bi-functionalcatalyst layer onto the intermediate support layer, such as cyclicvoltammetry, constant current or pulsed current electrodeposition,constant potential or pulsed potential deposition or combinationsthereof. Other embodiments of the gas diffusion negative electrodemanufacturing could include electrodeposition of the non-PGM oxideintermediate layer and/or spraying of the catalyst layer using an inkcontaining the bi-functional catalyst material, carbon and PTFEadditives

Experimental Tests

The following experiments demonstrate that a metal-free CO₂ redox flowbattery as a single unit can perform as an energy storage system withdirect CO₂ utilization.

Preparation of Catalysts and Gas Diffusion Electrodes (GDE)

A 40 wt % PTFE-treated carbon fiber cloth from Fuel Cell Earth was usedas the substrate for the negative electrode preparation. The substratewas subjected to a HNO₃ solution pre-treatment to increase the surfaceroughness and wettability before the electrodeposition step. Theelectrodeposition baths were composed of various combinations andconcentrations of palladium (II) chloride (PdCl₂, Sigma-Aldrich, 99%)(0-40 mM), tin (II) chloride (SnCl₂, Sigma-Aldrich, 98%) (0-40 mM), lead(II) acetate tri-hydrate (Pb(CH₃CO₂)₂.3H₂O, Sigma-Aldrich, ≥99%) (0-20mM), Indium (III) sulfate (In₂(SO₄)₃, Sigma-Aldrich, ≥98.0%) (0-20 mM),hydrochloric acid (HCl, Sigma-Aldrich, 37%) and Triton X-100 (EMDMillipore Corporation). Using a three-electrode electrochemical setup, awide range of catalysts including: Pd, Sn, PdSn, PdSnPb and PdSnIn, wereelectrodeposited on the carbon substrate by a cyclic voltammetrytechnique applied for 50 cycles between −0.3 and 1 V_(Ag/AgCl) at 0.02 Vs⁻¹ followed by pulsed potentiostatic deposition for 6 cycles at −1V_(Ag/AgCl) (4 min) with break at open-circuit potential (1 min) duringeach cycle. The reference and counter electrodes were Ag/AgCl withsaturated KCl (E_(e,298K)=0.199 V_(SHE), Cole-Parmer) and perforatedplatinized titanium plate, respectively. Following the electrodepositionprocedure, the samples were cleaned with isopropyl alcohol at 343 K for15 min to wash off any surfactant residues from the electrodepositedlayer.

For the oxide supported catalyst samples, non-Platinum Group Metal(non-PGM) oxides, MnO₂ (Sigma-Aldrich), LaCoO₃ (synthesized viaco-precipitation method) or MnO₂—LaCoO₃ (1:1 weight ratio), were sprayedas an ink mixture with Vulcan XC-72R prior to the electrodepositionprocedure on the pretreated carbon fiber cloth.

Surface and Crystallographic Characterization of the Catalysts

A field emission scanning electron microscopy (FESEM, Hitachi S-4700)equipped with an energy dispersive X-ray (EDX) gun was employed toperform EDX mapping, elemental analysis and morphological observationsof the electrodeposited catalysts. X-ray diffraction (XRD, RigakuMultiFlex) was employed to analyze the crystallographic structures ofthe electrodeposited catalysts (XRD conditions: generator set at 40 kVand 40 mA; Cu as X-ray source; wavelength of 1.541874 Å K_(α1); scanrate 1° (2θ) per minute).

Formate Analysis

The formate concertation in the solutions was analyzed using bothspectrophotometric and ion chromatography methods. The net formatefaradaic efficiency and net formate production rates presented in thiswork are cumulative for each data point during the experiments.

Half-Cell Setup for Electrocatalysis Studies

A flooded-cell setup was employed to test the electrocatalytic activityof electrodeposited catalysts for both CO₂ reduction (CO₂RR) and formateoxidation (FOR) reactions. A conventional three-electrode cellconfiguration was used with the catalyst deposited electrode fitted intoa rotating disk electrode (RDE) set up as a working electrode (geometricarea=0.28 cm²), Ag/AgCl KCl saturated reference electrode and a spiralplatinum wire as a counter electrode. A range of electrochemical testswere performed including cyclic voltammetry (CV), chronopotentiometry(CP) and galvanostatic cycling. A divided cell with the positiveelectrode compartment separated by a Nafion 115 membrane from theworking (i.e., negative) electrode was used to measure the net formatefaradaic efficiency for the catalysts in a flooded-cell setup. Allelectrode potentials are reported versus the Ag/AgCl KCl saturatedreference electrode unless otherwise specified or labeled.

Preliminary Battery Cell Design

The batch-type battery tests were performed in a divided cell with twoL-shaped electrode holders protruding toward the membrane on each sidein order to decrease the distance between negative and positiveelectrodes, hence, lower the cell resistance. On the negative electrodeside, CO₂ gas was purged at the back of the bi-functional GDE (geometricarea=1.33 cm²) through the L-shaped holder at a flow rate of 3.17×10⁻³standard liter min⁻¹ (SLM) at 1 atm pressure. The negolyte was either 2M KHCO₃ (for state-of-charge tests) or 2 M KHCO₃+1 M KHCO₂ (forcharge-discharge polarization and galvanostatic cycling tests) facingthe GDE. The negative electrode assembly included a stainless steel meshas a current collector, two Freudenberg H23C7 teflonated carbon papers(thickness 250 μm each) acting as gas flow distributor. The GDE preparedas described previously was pressed against the gas flow distributor onone side and faced the negolyte solution on the other side. The GDEcontained either two or three layers, whether the catalyst isunsupported or supported, respectively: 1) a 40 wt % PTFE-treated carbonfiber cloth (from Fuel Cell Earth, thickness 380 μm) as thebacking/substrate layer, 2) a sprayed layer of Vulcan XC-72R and anon-PGM oxide powder (i.e., MnO₂, or LaCoO₃ or MnO₂—LaCoO₃ with theloading of 0.5 mg cm⁻² each) at a weight ratio of 1:1 acting as thecatalyst support layer, and 3) the electrodeposited PdSn or PdSnPbcatalyst layer. The electrodeposition procedure was previously describedin the GDE preparation section.

A 40 wt % PTFE-treated carbon fiber cloth (from Fuel Cell Earth) wasused as a positive electrode (geometric area=1.33 cm²) in 0.3 M Br₂+2 MKBr (posolyte) to complete the battery cell. A cation exchange membrane(Nafion® 115) was employed to transport K⁺ from the positive to thenegative electrode compartment during battery charge and in the oppositedirection during discharge. In the CRB, it is essential that the cationexchange membrane minimizes any anion anion crossover (including bromideand formate), hence, no anion exchange membrane can work for thissystem. The membrane was treated for 1 hr in 5 wt % H₂O₂ at 343 Kfollowed by 20 min in DI water at 298 K, 1 hr in 0.5 M H₂SO₄ at 343 K,20 min in DI water at 298 K and 1 hr in 0.5 M KOH at 313 K prior to theexperiments. The battery polarization tests were run at 0.1 mA s⁻¹starting with the discharge polarization cycle followed by a 15 min.long open circuit potential (OCP) measurement and followed by the chargepolarization cycle. The galvanostatic cycling experiments in the batterysetup were performed at 0.5 mA cm⁻².

(a) Synthesis, Characterization and Bi-Functional CO₂ Reduction (CO₂RR)and Formate Oxidation Reaction (FOR) Electrocatalytic Activities of PdSnCatalysts

Referring to FIGS. 2(a)-(d), catalyst formulations for the batterynegative electrodes were based on PdSn due to the well-establishedindividual activities of Sn for CO₂RR and Pd for FOR, respectively.Therefore, it was of interest to investigate whether the combination ofthese two elements could provide the necessary bi-functionalperformance. PdSn catalysts were prepared by electrodeposition onto ateflonated (40 wt %) carbon fiber cloth according to the proceduredescribed above. Structural characterization reveals the formation ofspherical deposits with a dual structure, composed of larger sphericalaggregates (˜1 μm diameter) decorated with a network of nanoparticles(˜10-100 nm diameter) (FIG. 2(a)). The EDX mapping shows a homogeneousdistribution of Pd and Sn with a 2:1 atomic ratio throughout thedeposited structures (see FIG. 2(b)). Moreover, XRD analysis reveals avariety of intermetallic compositions including PdSn, PdSn₂, PdSn₃,Pd_(0.83)Sn_(0.17), as well as the presence of individual (unalloyed)Pd, Sn and SnO₂ (see FIG. 2(c)).

The cyclic voltammetry behavior of pure Pd and the electrodeposited PdSnwere compared in the absence and presence of CO₂. Referring to FIG.2(d), cyclic voltammograms are shown of Pd (left) and PdSn (right)samples in CO₂ saturated 0.5 M KHCO₃+0.1 M KHCO₂ (pH of 7.4) and N₂saturated 0.5 M KHCO₃ (pH adjusted to 7.4), with catalyst loadings of Pdat 10.7 mg cm⁻² and PdSn at 6.6 mg cm⁻², 10 Cycles, 293 K., 2000 rpm. Inthe formate-free N₂ saturated electrolyte, the cyclic voltammogram of areference pure Pd electrodeposited sample presents an anodic wave foroxide formation (around 0.75 V), a reduction peak on the cathodic sweepat −0.14 V for PdO_(x) reduction and another reduction peak at −1.2V dueto hydrogen adsorption followed by the hydrogen gas evolution reaction(HER) at more negative potentials (FIG. 2(d)). The addition of 0.1 Mformate to the electrolyte generated two characteristic formateoxidation waves; one on the forward anodic sweep (peak potential 0.9 V)and the other one with higher oxidation peak current density on thereverse cathodic sweep. The latter wave (peak potential at 0.5 V) ischaracteristic for the direct two-electron formate oxidation on theCO-free and partially oxidized surface (see FIG. 2(d)). In the CO₂saturated electrolyte, the reduction currents are in the potentialdomain where CO₂RR is expected to occur at high rate (i.e., potentiallower than −1.2 V vs. Ag/AgCl, KClstd.) are similar to those in the N₂saturated electrolyte at the same pH (7.4). This finding suggests thatpure Pd is not a bi-functional catalyst and in the latter potentialrange only HER takes place. Based on literature, Pd surfaces were shownto have only minor activity for CO₂RR to CO and CH_(x) at high cathodicoverpotentials, therefore, it is clear pure Pd is not a suitablecatalyst for the CRB.

In contrast, the intermetallic PdSn catalyst shows higher cathodiccurrent densities in the presence of CO₂ compared to N₂ at potentialslower than −1.6 V (FIG. 2(e)) due to Sn and its well-known activity forCO₂RR to formate. PdSn is also active for FOR with the corresponding FORpeak potential on the cathodic scan approximately 0.35 V lower on PdSncompared to pure Pd (FIG. 2e vs. 2 d). This indicates that in the caseof PdSn the unoxidized surface is the most active for FOR catalysis. Theanodic passivation of Sn forming SnO_(x) at high potentials (>0.7 V vs.Ag/AgCl KClstd.), renders the surface less active for FOR. The SnO_(x)layer on the other hand, can play a major beneficial role for CO₂RR toformate. Thus, in the case of bi-functional catalysis, there is a stronginterdependency between the two different potential domains of operationwith respect to the state of the catalyst surface. On a mass activitybasis, the FOR peak current per catalyst mass on the cathodic scan isvirtually identical for PdSn and Pd (FIGS. 2(d) and (e)), indicatingthat the presence of Sn does not hinder the FOR.

Overall, based on cyclic voltammetry experiments the PdSn catalystdemonstrated bi-functional electrocatalytic activity for CO₂RR and FORin the same electrolyte and pH (FIG. 2(e)). Further validation ispresented in the next sections.

(b) CO₂RR and FOR Cycling Durability and Non-Platinum Group Metal(Non-PGM) Oxide Supports for PdSn Catalyst

Referring to FIGS. 3(a) to (d), the effect of non-PGM supports on theelectrocatalytic activity of bi-functional PdSn catalysts were tested,wherein: FIG. 3(a) involves galvanostatic polarization cycling betweenCO₂RR and FOR of electrodeposited PdSn catalysts, FIG. 3(b) involvesonly teflonated carbon fiber cloth (no non-PGM support), and FIG. 3(c)involves a LaCoO₃ support (loading of 0.5 mg cm⁻²) on teflonated carbonfiber cloth. The sequence started with 5 min. of OCP measurement,followed by FOR at 50 mA cm⁻² and then CO₂RR at −35 mA cm⁻², each for 5min., with FOR—CO₂RR cycle repeating for 6 times in CO₂ saturated 0.1 MKHCO₂+0.5 M KHCO₃. FIGS. 3(b) and (c) show cyclic voltammograms of PdSncatalyst in the FOR potential region after 1 hr of FOR—OCP at 50 mA cm⁻²in 0.1 M KHCO₂+0.5 M KHCO₃ followed by 1 hr of CO₂RR at −35 mA cm⁻² inCO₂ saturated 0.5 M KHCO₃. FIG. 3(e) are representative cyclicvoltammograms of wide potential range electrochemical activation appliedto the PdSn with LaCoO₃ support consisting of 50 cycles in N₂ saturated0.5 M KHCO₃ starting from −2 V to 1.1 V at 0.02 V s¹. FIG. 3(f) showsgalvanostatic polarization cycling of the activated LaCoO₃supported-PdSn catalyst with the FOR—CO₂RR tests repeating for 18cycles, with a PdSn loading:2.7 mg cm⁻², at 293 K and 2000 rpm.

Cycling durability between CO₂RR and FOR is an essential requirement forthe negative electrode catalyst of the CRB. Galvanostatic polarizationcycling between −35 and +50 mA cm⁻² of the intermetallic PdSn catalystelectrodeposited on teflonated carbon fiber cloth, presented andcharacterized in the previous section, shows potential oscillationsduring FOR after only three cycles (FIG. 3a ). These oscillationstrigger a corresponding dramatic loss of FOR activity as shown by thecomplete absence of the formate oxidation wave and peak in the cyclicvoltammogram following the one-hour galvanostatic cycling protocol (FIG.3b ). It is important to note that the FOR oscillations also occur whenthe galvanostatic polarization cycling does not extend to the CO₂RRpotential region. Hence, it is exclusively related to FOR. Deactivationof FOR with cycling is likely caused by poisoning adsorbates such asCO_(ad) formed through the indirect FOR pathway, and possibly assistedby OH⁻. The CO₂RR activity, on the other hand, is stabilized after fivecycles at a potential of approximately −0.95 V (FIG. 3(a)).

To overcome the FOR deactivation and enhance the bi-functionaldurability of the intermetallic PdSn catalysts, we investigated non-PGMoxides, i.e., LaCoO₃, MnO₂, MnO₂—LaCoO₃ as an intermediate layer betweenthe porous teflonated carbon substrate base layer and the catalystlayer. The non-PGM oxides alone or in combination have noelectrocatalytic activity on their own toward FOR and poor activity forCO₂RR compared to PdSn. Thus, these oxides act mainly as catalystsupports.

Cyclic voltammograms show that the non-PGM oxide supports have virtuallyno effect on the CO₂RR behavior of PdSn compared to the teflonatedcarbon fiber cloth substrate but they have a strong impact on FOR.Furthermore, cyclic galvanostatic polarization experiments revealsuperior CO₂RR and FOR electrocatalytic activities and FOR stability forthe LaCoO₃-supported PdSn, as shown by the elimination of potentialoscillations and deactivation in the FOR region (compare FIG. 3(c) with3(a) and FIG. 3(d) with 3(b)). Among the investigated oxide supports,while MnO₂ eliminated the FOR oscillations as well, LaCoO₃ showedoverall better performance. Based on literature analysis, it is proposedthat the LaCoO₃ support effect could be due to the ability of theperovskite structure to adsorb and convert CO_(ad), a poisoningintermediate formed during FOR, via a heterogeneous catalytic reactionThus, CO adsorbs on and reacts with the perovskite lattice oxygen (i.e.,O²⁻ ions) or weakly-bonded adsorbed oxygen to form CO₂ which is desorbedfrom the surface leaving behind a CO_(ad)-free surface on the LaCoO₃support. In this mechanism, therefore, it is proposed that theperovskite support provides active sites for CO_(ad) and oxidation toCO₂. It is further hypothesized that there could be an interminglingbetween the metallic catalyst and support atoms (i.e., Pd and La,respectively) such that CO_(ad) formed on the catalyst sites can easilysurface diffuse to the perovskite lattice oxygen sites.

Next, further gains in bi-functional catalytic activity can be obtainedby carrying out a wide potential range electrochemical activation of thePdSn catalysts. The activation protocol consisted of fifty cycles from−2 V to 1.1 V at 0.02 V s⁻¹ in N₂ saturated 0.5 M KHCO₃ solution (FIG.3(e)). As a result of the activation cycles, distinguished oxidation andreduction peaks appear in the PdSn voltammograms corresponding to Snoxidation (around −0.41 V) and oxide reduction (around −0.43 V),respectively, with a minor peak for Pd oxidation (starting at 0.32 V).The shifts in the location of these oxidation and reduction peaks havebeen associated with successive formation and reduction of Pd/Sn oxideswith complex composition. Moreover, the surface oxidation/reductioncycles (particularly with regard to SnO_(x)/Sn) carried out during theelectrochemical activation of PdSn, generate more negative currentdensities at potentials lower than −1 V due to the formation of nascentmetallic Sn surface with higher surface area and increased HER activity(FIG. 3(e), compare cycle 1 and 50).

Comparing the galvanostatic cycling results of the unactivated (FIG.3(c)) and activated (FIG. 3(f)) LaCoO₃ supported PdSn, theelectrochemical activation enhances both the CO₂RR and FOR performanceas shown by the smaller potential difference between the FOR and CO₂RRpotentials. However, after the first three cycles, the effect is morepronounced for CO₂RR, in which case up to 0.5 V decrease in (absolute)value of CO₂RR potentials is observed at −35 mA cm⁻² (compare FIGS. 3fand 3c ). Moreover, the activated and perovskite supported catalystshows a very stable behavior during 3 hrs. of galvanostatic cycling(FIG. 3f ). The enhanced electrocatalytic activity and durability ofactivated PdSn catalysts are likely due to the formation of mixed-oxidessuch as SnO_(x)/Sn during the wide potential range electrochemicalactivation characterized by high surface area and enhancedelectrocatalytic activity toward CO₂RR to formate in mild alkalinemedia.

(c) Ternary Electrocatalyst Formulations for CO₂RR—FOR Bi-FunctionalActivity

Referring to FIGS. 4(a) to (d), bi-functional performance between binaryand ternary catalysts were tested, wherein FIG. 4(a) shows galvanostaticpolarization cycling of the LaCoO₃ supported PdPbSn and PdSnIn catalystswith electrochemical activation applied; FIG. 4(b) shows galvanostaticpotentials for CO₂ electro-reduction at superficial current densities of−20, −35 and −50 mA cm⁻² on activated bimetallic and ternaryelectrocatalyst (time 30 min.); and FIGS. 4(c) to (e) show formateproduction faradaic efficiency (FE) and net formate production rates ongeometric electrode ara and catalyst mass basis respectively. Thecatalyst loadings: PdSn 1.4 mg cm⁻², PdPbSn 8.1 mg cm⁻² and PdSnIn at3.6 mg cm⁻². All other conditions idem to those provided above for thetests shown in FIG. 3.

Further bi-functional catalyst development focused on ternaryformulations containing In and Pb in addition to PdSn. Both Pb and Inwere chosen for co-electrodeposition along with Pd and Sn mainly due totheir electrocatalytic activity for CO₂ reduction to formate. Themorphology of the ternary catalysts is composed of compact sphericalaggregates (less than 2 μm), with uniform distribution of all threeelements as shown by EDX mapping. In terms of elemental composition, thePd:Sn:Pb atomic ratio was 1:1.2:0.1, while Pd:Sn:In atomic ratio was1:0.32:0.07. The XRD spectra show that PdPbSn has a crystallinestructure containing a combination of intermetallics such as PdSn₂,Pb₉Pd₁₃, Pd₃Sn as well as Pd, Sn and Pb alone (FIG. S9 b). In contrast,PdSnIn has an amorphous structure.

FIG. 4 compares the bi-functional electrochemical performance of theternary catalysts (PdSnPb, PdSnIn) with LaCoO₃ intermediate supportlayer and electrochemical activation applied (as described in theprevious section). The galvanostatic cycling profiles, electrodepotentials, net formate production rates and formate Faradaicefficiencies are shown as a function of superficial current density.Ideally, such a comparison should take into account theelectrocatalytically active surface area (ECSA). The real currentdensity (i.e., normalized per ECSA) is the best indicator of theintrinsic catalytic activity for different catalyst compositions.Determining the ECSA for PdSnPb, PdSnIn and PdSn is not trivial sincethese surfaces do not respond well to commonly used methods such asunderpotential hydrogen deposition or CO_(ad) stripping. In light ofthese observations, the comparisons presented in FIG. 4 must beconsidered on a relative basis, not necessarily an absolute reflectionof the intrinsic, catalyst-composition-dependent, activity.

Among the catalysts investigated, the galvanostatic polarization cyclingtests (compare FIGS. 3(f) and 4(a)) reveal that at the same superficialcurrent densities, the activated-PdPbSn/LaCoO₃ is slightly betterperforming than both activated-PdSn/LaCoO₃ and activated-PdSnIn/LaCoO₃.For the PdPbSn, the CO₂RR and FOR potentials are −1.1 V (at −35 mA cm⁻²,65 min.) and 0.28 V (at 50 mA cm⁻², 60 min.), respectively.

Chronopotentiometry tests in a batch-divided cell setup corroborate thegalvanostatic cycling results in that the LaCoO₃ intermediate layersupported and activated PdSnPb and PdSn provide the highestCO₂-to-formate faradaic efficiencies (FEs) and lowest (in absolutevalue) CO₂RR potentials at a constant current density (FIGS. 4b and c ).The highest FEs were between 66% and 73% (at −20 and −35 mA cm⁻²) forthe activated PdSnPb. The FE of CO₂-to-formate was extensively studiedin the literature under a variety of conditions (i.e., catalysts,cathode potentials, cathode superficial current densities, electrolytecomposition, etc.). FE values as high as 95% were reported for Sn or Pbor SnPb. The lower formate generation FEs shown in FIG. 4(b) could arisefrom three possible sources. First, the incorporation of Pd in thecatalyst layer is not beneficial to formate generation since Pd mightnot be catalytically active for reduction to formate. Second, thelimited solubility of CO₂ in aqueous electrolytes (i.e., 34 mM at 1 atmand 298 K in distilled water) can lead to CO₂ starvation of the catalystat high current densities (e.g., at −50 mA cm⁻²) during prolonged tests.Third, formate losses due to crossover through the Nafion membrane tothe counter electrode compartment. The thermo-catalytic dehydrogenationof formate on Pd has been discarded as a possible mechanism for formateloss due to sluggish kinetics at temperatures below 353 K.

Furthermore, in terms of net formate production rate on a geometricelectrode area basis, the activated PdSnPb catalyst on LaCoO₃ supporthas shown the highest rate of 22.7 μM cm⁻² min⁻¹ (7.9 μmol cm⁻² min⁻¹)at −35 mA cm⁻² during 30 min. of testing (FIG. 4d ), which is at least15% higher than the production rates reported for Sn, Pb, SnPbcompounds, In, Zn and bimetallic In—Zn nanocrystals in similar testsetup conditions. However, when the formate production is normalized percatalyst mass, clearly the activated and LaCoO₃ supported PdSn is thebest performing sample among those investigated here (FIG. 4e ).Therefore, perovskite supported and activated PdSn and PdSnPb,respectively, were retained for battery experiments.

(d) CO₂ Redox Flow Battery (CRB)

Referring to FIG. 5, and in order to demonstrate the polarizationbehavior of the CO₂ redox battery a preliminary cell design was usedwith continuous flow of CO₂ gas to GDE coupled with batch liquidelectrolytes, negolyte and posolyte, respectively. The custom madeL-shaped electrode holder delivers the CO₂ gas to the tip holdingtightly together a stainless steel current collector mesh, twoteflonated carbon papers with embedded intermediate layers (FreudenbergH23C7) acting as gas distributors, and the GDE containing thebi-functional catalyst layer (FIG. 1c and FIG. 5). Themembrane-electrode inter-spacing is about 5 mm on each side. Anobjective with this design was to develop a cell that allows fast andeasy screening of different negative and positive electrode catalystsand membranes at different temperatures.

FIG. 6 compares the CRB discharge and charge performance for PdSnnegative electrode catalyst with and without perovskite support and/orelectrochemical activation. The polarization curves in FIG. 6 are allohmic potential (or IR) drop-corrected to eliminate membrane andelectrolyte resistivity effects. The open-circuit potential of the CRBis approximately 1.5 V, in very good agreement with the calculatedequilibrium potential based on eqns. (1) to (6). The positive Br₂/Br⁻electrode has comparatively faster electrode kinetics, hence, thekinetic limitations of the battery can be mostly attributed to thenegative battery electrode. The perovskite support and electrochemicalactivation presented in the previous sections are essential fordecreasing the negative electrode activation overpotentials duringbattery discharge and charge, respectively. As a result, for theperovskite-supported and activated catalyst the discharge currentdensity range is almost five times higher compared to the referenceunsupported and unactivated PdSn (FIG. 6). For the charge step as well,the lower activation overpotential on the negative electrode with theperovskite-supported and activated catalyst, translates in lowering thebattery charge voltage required at the same current density (FIG. 6).

A comparison of the battery performance with binary PdSn and ternaryPdSnPb catalysts (both catalyst types are perovskite supported andelectrochemically activated, as previously described), is shown by FIG.7 with both IR-corrected and uncorrected (i.e., original) polarizationcurves. The CRB equipped with the LaCoO₃ supported and activated PdSnGDE produced the highest peak power densities at either 293 K or 318 K(FIG. 7). The peak power density obtained with PdSn is similar to thevalue obtained with Pd alone, but as discussed in the previous sections,with the Pd only catalyst the battery cannot be recharged (i.e., Pd isan unifunctional catalyst for formate oxidation only).

Furthermore, the discharge cell potential with PdSn dropped abruptly atsuperficial current densities higher than 15 or 20 mA cm⁻² (at 293 K and318 K, respectively), in a manner reminiscent of mass transfer control.However, such an abrupt cell potential drop was not observed with eitherpure Pd or PdSnPb. This suggests that instead of mass transferlimitation the abrupt cell potential drop could be due to deactivationof specific PdSn sites. As discussed previously with regard to FIG. 2e ,at high anode potentials during discharge (translating to battery cellpotentials 0.9 V, FIG. 6), SnO_(x) and/or Pd_(x)Sn_(y)O_(z) are formedon the catalyst surface with lower activity toward formate oxidation(due to anodic passivation of Sn). On the other hand, this SnO_(x) layeris beneficial for CO₂RR to formate in the subsequent battery chargingstep as shown by the low battery charging voltage of about 1.65 V up to20 mA cm⁻². At higher charging current densities (>20 mA cm⁻²) the oxidelayer is reduced generating a jump of about 0.3 V in the battery chargevoltage (FIG. 7, PdSn catalyst case) since CO₂RR to formate is lessefficient on the reduced (metallic) surface. A similar jump in thecharge voltage is observed for pure Sn as well, albeit it occurs at alower charge current density (˜5 mA cm⁻²).

When Pb was added to the catalyst formulation, the discharge currentdensities could be increased two to three times compared to PdSn butwith the drawback of lower power density (FIG. 7). Interestingly, PdSnPbwas also much less sensitive to temperature increase (from 293 K to 318K) compared to PdSn especially at superficial current densities higherthan 10 mA cm⁻² (FIG. 7). This suggests that for the latter catalyst,the FOR mechanism could be different and steps other than electrontransfer kinetics could be rate-limiting. Further studies are requiredto better understand these effects.

Looking at other emerging non-metal or CO₂ battery technologies (Table1), the CRB with the preliminary batch-cell design presented here,provided four to nineteen times higher peak power densities compared toS-air and CO₂/CH₄—Zn, respectively. This clearly signifies the superiorperformance and transformative potential of the CRB as an emergingCO₂-utilizing, metal-free battery.

TABLE 1 Peak performance comparison of selected emerging new non-metaland CO₂ battery technologies Voltaic round-trip efficiency Open- Peakdischarge power (and energy circuit cell density (mW cm⁻²) @ efficiency)potential current density at peak T Battery type (V) (mA cm⁻²) power (%)(K) CRB 1.5 21.6* @ 20.5    61%* 318 19.2 @ 20.5 (44.5%)* 50.5% (36.7%)  S-air 1.5 5.1 @ 7.1 <10%  328 (NA) CO₂/HCOOH—Zn 0.89 5.8 @ 0.5~40%  293 (NA) CO₂/CH₄—Zn 1 2.5 @ 1   NA 293

Referring to FIGS. 8(a)-(e), the charge/discharge capability of the CRBwith LaCoO₃-supported and activated PdSn GDE was further investigatedthrough state-of-charge and galvanostatic cycling experiments (FIGS.8(a) and 8(b). These results are compared to individual Sn and Pd GDEs,to further substantiate the bi-functionality. The activated PdSn had thelowest charge potential over three intermittent hours of charging at 20mA cm⁻², i.e., 2.2 V, with an open-circuit potential (OCP) gain of 1 Vduring the first charge reaching a value of about 1.5 V, which indicatessuccessful production of formate by CO₂ reduction. This corresponds to anet formate Faradaic efficiency of 73% and a net formate production rateof 18.25 μM cm⁻² min⁻¹ (4.56 μmol cm⁻² min⁻¹). The 64% increase in thenet formate formation rate for the PdSn catalyst in the current setupcompared to the flooded-cell experiments (FIG. 4) clearly shows thebenefit of a two-phase gas-aqueous system compared to the aqueous onlyelectrolyte with CO₂ gas solubility limitations. In terms of secondaryreactions during battery charge, mainly the H₂ evolution reaction (HER)accounts for the Faradaic efficiency difference between 73 and 100%.

Further, galvanostatic charge-discharge steps at ±0.5 mA cm⁻² wereapplied to the aforementioned catalysts for one hour of testing (FIGS.8b, 8d and 8e ). The applied current was chosen to avoid any possiblemass transfer limitations arising from the cell design with batchelectrolytes (FIG. 5). The activated and LaCoO₃-supported PdSn GDE wasthe most stable, cycling between 1.5 V (at charge) and about 1.2 V (atdischarge) with voltaic round-trip efficiencies (RTE) of 80-86% (FIG. 8b). At the low current densities of ±0.5 mA cm⁻² (FIG. 8(b)) the Faradaicefficiencies during charge and discharge are virtually 100%, hence, thevoltaic RTE is essentially equal to the energy RTE. With Sn alone, beinga catalyst for CO₂RR to formate, charging the battery to an OCP of 1.6 Vis possible (FIG. 8(c)), but the galvanostatic cycling demonstratesclearly that there is no discharge functionality due to the sharp cellpotential drop during discharge (FIG. 8(d)). Conversely, Pd alone can beonly charged to an OCP of approximately 1.3 V (FIG. 8(e)) at a highcharge voltage of 2.5 V. The Pd GDE cycles between 1.9 V (at charge) and0.8 V (discharge) with RTE of 41% following the 3^(rd) cycle (FIG. 8f ).Overall, neither Pd nor Sn alone have suitable bi-functionality for theCRB.

FIG. 9 shows the power density of the single-cell CRB when the CRBnegative electrode catalyst layer was prepared by spraying in contrastto electrodeposition presented in the previous examples. The teflonatedcarbon cloth base layer was sprayed first with a layer containing LaCoO₃(with a loading of 0.5 mg cm⁻²) as the intermediate oxide support layer(FIG. 10). The ink composition included a mixture of LaCoO₃:VulcanXC-72:Nafion:PTFE at a weight ratio of 1:1:0.6:0.63. The ink wassonicated for half an hour and then applied using a CNC-controlledsprayer machine on the carbon cloth at 45° C. The Vulcan XC-72 carbonblack in the oxide intermediate layer can be replaced with othercarbon-based materials alone or in combination such as: graphene,graphene oxide, carbon nanotube, Ketjenblack, graphitized carbon. TheNafion ionomer in the oxide intermediate layer can be replaced by othermembrane ionomers with cation exchange, bipolar or anion exchangeproperties. After the intermediate layer was sprayed, the bi-functionalcatalyst layer containing Pd/C and SnO₂ powders mixed at a weight ratioof 1: was sprayed as well using the CNC-controlled sprayer on top of theintermediate layer such that to obtain a loading of 2 mg cm⁻² each ofPd/C and SnO₂, respectively. The ink composition for spraying thebi-functional catalyst layer contained in addition to Pd/C and SnO₂,PTFE, Nafion and Vulcan XC72. In one of the examples, the finalcomposition of the bi-functional catalyst layer consists of: 26.5 wt. %Pd/C, 26.5 wt. % SnO₂, 26.5 wt. % Vulcan XC-72, 16.7 wt. % PTFE and 4wt. % Nafion. The Vulcan XC-72 carbon black in the catalyst layer can bereplaced with other carbon-based materials alone or in combination suchas: graphene, graphene oxide, carbon nanotube, Ketjenblack, graphitizedcarbon. The Nafion ionomer in the catalyst layer can be replaced byother membrane ionomers with cation exchange, bipolar or anion exchangeproperties.

Comparing the electrodeposited and sprayed catalysts, it is concludedthat the electrodeposited PdSn catalyst provides superior discharge andcharge polarization performances for the CO₂ Redox Flow Battery (CRB).However, the spraying method, compared to the electrodeposition process,is easier and more feasible to scale up. Moreover, the sprayed catalysthas shown better durability.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. Accordingly, asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises” and“comprising,” when used in this specification, specify the presence ofone or more stated features, integers, steps, operations, elements, andcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components, andgroups. Directional terms such as “top”, “bottom”, “upwards”,“downwards”, “vertically”, and “laterally” are used in the followingdescription for the purpose of providing relative reference only, andare not intended to suggest any limitations on how any article is to bepositioned during use, or to be mounted in an assembly or relative to anenvironment. Additionally, the term “couple” and variants of it such as“coupled”, “couples”, and “coupling” as used in this description areintended to include indirect and direct connections unless otherwiseindicated. For example, if a first device is coupled to a second device,that coupling may be through a direct connection or through an indirectconnection via other devices and connections. Similarly, if the firstdevice is communicatively coupled to the second device, communicationmay be through a direct connection or through an indirect connection viaother devices and connections.

It is contemplated that any part of any aspect or embodiment discussedin this specification can be implemented or combined with any part ofany other aspect or embodiment discussed in this specification.

The scope of the claims should not be limited by the preferredembodiments set forth in the examples, but should be given the broadestinterpretation consistent with the description as a whole.

What is claimed is:
 1. A redox flow battery comprising: (a) a negativeelectrode comprising a porous base layer, a bi-functional catalyst layerfor electrochemical reduction of either CO₂ or carbonate to formateduring battery charging and for formate oxidation to either carbonate orCO₂ during battery discharge, and an intermediate support layersupporting the bi-functional catalyst layer and comprising a metaloxide; wherein the metal oxide comprises: a perovskite structure withthe formula AxByO₃, wherein A is one or a mixture of La, Sr, and Ba andB is one of Co, Ti, Fe, Ni, Ga, Mg, In, Mn, Ta, or Ce; or a fluoritestructure with the formula AxByO₇, wherein A is Nd, and B is Ir; (b) apositive electrode; and (c) a cation exchange or bipolar membrane inbetween the negative and positive electrodes.
 2. The redox flow batteryas claimed in claim 1, wherein the bi-functional catalyst layercomprises one or more of: Pd, Sn, an intermetallic species with theformula Pd_(x)Sn_(y) and SnO₂.
 3. The redox flow battery as claimed inclaim 2 wherein the bi-functional catalyst layer further comprises In orPb.
 4. The redox flow battery as claimed in claim 2 wherein thebi-functional catalyst layer is electrodeposited on the intermediatesupport layer.
 5. The redox flow battery as claimed in claim 4 whereinthe bi-functional catalyst layer comprises polytetrafluoroethyelene(PTFE) and one or more carbon additives selected from a group consistingof: carbon black, graphene, graphene oxide, reduced graphene oxide,graphitized carbon and carbon nanotubes, with a PTFE to carbon additiveweight ratio between 0.1:1 to 5:1.
 6. The redox flow battery as claimedin claim 2 wherein the bi-functional catalyst layer is an ink sprayed onthe intermediate support layer.
 7. The redox flow battery as claimed inclaim 6 wherein the bi-functional catalyst layer comprisespolytetrafluoroethyelene (PTFE) and one or more carbon additivesselected from a group consisting of: carbon black, graphene, grapheneoxide, reduced graphene oxide, graphitized carbon and carbon nanotubes,with a PTFE to carbon additive weight ratio between 0.1:1 to 5:1.
 8. Theredox flow battery as claimed in claim 7, wherein the intermediatesupport layer comprises LaCoO₃ electrodeposited onto the porous baselayer.
 9. The redox flow battery as claimed in claim 8 wherein theintermediate support layer further comprises silicon with the formulaAxBySiO₄, wherein A is one of Mg, Zr, and Ca, and B is one of Fe and Ni.10. The redox flow battery as claimed in claim 8 wherein the metal oxidein the intermediate support layer comprises one of Ce, Zr, Al, and Ga.11. The redox flow battery as claimed in claim 7 wherein theintermediate support layer comprises LaCoO₃ ink sprayed on the porousbase layer.
 12. The redox flow battery as claimed in claim 11 whereinthe porous base layer is a teflonated carbon cloth or a carbon fiberpaper.
 13. The redox flow battery as claimed in claim 12 wherein theintermediate support layer comprises polytetrafluoroethylene (PTFE) andcarbon additives selected from a group consisting of: carbon black,graphitized carbon, graphene, graphene oxide, reduced graphene oxide,and carbon nanotubes, with a PTFE to carbon additive weight ratiobetween 0.1:1 to 5:1.
 14. The redox flow battery as claimed in claim 13wherein the intermediate support layer comprises LaCoO₃ mixed with MnO₂.15. The redox flow battery as claimed in claim 1, wherein theintermediate support layer comprises LaNiO₃ electrodeposited on theporous base layer.
 16. The redox flow battery as claimed in claim 1,wherein the intermediate support layer comprises LaNiO₃ ink sprayed onthe porous base layer.
 17. The redox flow battery as claimed in claim 1,wherein the intermediate support layer comprises LaNiO₃ mixed with MnO₂.18. A method for electrochemically activating a negative electrode of aredox flow battery that comprises the negative electrode, a positiveelectrode, and a cation exchange or bipolar membrane in between thenegative and positive electrodes, the negative electrode comprising aporous base layer, a bi-functional catalyst layer for electrochemicalreduction of either CO₂ or carbonate to formate during battery chargingand for formate oxidation to either carbonate or CO₂ during batterydischarge, and an intermediate support layer supporting thebi-functional catalyst layer and comprising a metal oxide, the metaloxide comprising: a perovskite structure with the formula AxByO₃,wherein A is one or a mixture of La, Sr, and Ba and Bis one of Co, Ti,Fe, Ni, Ga, Mg, In, Mn, Ta, or Ce; or a fluorite structure with theformula AxByO₇, A is Nd, and B is Ir; the method comprising: electrodepotential sweeping between reduction and oxidation potentials or currentpulsing between reduction and oxidation currents.
 19. A method formanufacturing a bi-functional negative electrode for a redox flowbattery, comprising: (a) providing a porous carbon base layer; (b)providing a deposition mixture for an intermediate support layercomprising a metal oxide material having: a perovskite structure withthe formula AxByO₃, wherein A is one or a mixture of La, Sr, and Ba andB is one of Co, Ti, Fe, Ni, Ga, Mg, In, Mn, Ta, or Ce; or a fluoritestructure with the formula AxByO₇, wherein A is Nd, and B is Ir; (c)providing a deposition mixture for a bi-functional porous catalyst layercomprising a bi-functional catalyst; and (d) depositing byelectrodeposition or mechanical spraying the intermediate support layerdeposition mixture onto the carbon base layer followed by depositing byelectrodeposition or mechanical spraying the catalyst layer onto theintermediate support layer creating a metal oxide supported catalyst.